An electric motor's power is proportional to its torque and rotational speed. Increasing one or both of these properties increases the motor's power (i.e., its horsepower).
An electric motor has two basic components: a stator and a rotor. At least one of these components includes magnets, whose magnetic fields cause the rotor to move relative to the stator, usually by rotating about a central shaft.
The stator is the stationary electrical component. It typically includes a group of individual permanent magnets or electro-magnets arranged in a way to form a hollow, generally cylindrical shell with one pole of each magnet facing toward the center of the group.
The rotor is the rotating electrical component and typically comprises a generally cylindrical body mounted to a central shaft. The rotor includes a second group of magnets which are arranged around the outer surface in close proximity to the stator's magnets. The interaction between the magnetic fields of these magnets causes Lorentz forces to be exerted upon the rotor causing the rotor to turn, which causes the motor's output shaft to rotate. The distance between the rotor's magnets (where the Lorentz forces are directed against the rotor) and the output shaft represents the torque arm length of the motor. That is, the radius of the rotor determines the amount of torque applied to the output shaft by a given magnetic field.
Currently, in order to increase motor torque the Lorentz forces are increased by using larger magnetic surfaces and/or using more electrical power. While these systems will produce greater amounts of torque, they increase the motor's cost, size and weight.
Increasing the motor's rotational speed, while increasing the motor's power output, generally causes the rotor to vibrate or otherwise become unstable. One example of this instability is that the rotor will deflect or cant along its axis. This deflection and vibration limits how fast a particular motor may be run. One cause of this undesirable deflection at higher rotational speeds is that the support members/torque arms of conventional rotors typically project orthogonally from the rotor shaft and do not have adequate support along the rotational axis (along the rotor shaft).
Similarly, increasing the torque arm length of conventional motors requires the rotational speed to be reduced in order to avoid instability. By lowering rotational speed, the power benefits of the increased torque are offset of the lowered rotational speed.